KCSAN: data-race in task_mem / unmap_region

[Kaia Yadira]

Hello,

When using Syzkaller to fuzz the latest Linux kernel, the following
crash was triggered.

HEAD commit: a7904a538933 Linux 5.16-rc6
git tree: upstream
console output: KCSAN: data-race in task_mem / unmap_region
kernel config: https://paste.ubuntu.com/p/QB39MJKWKb/plain/
Syzlang reproducer: https://paste.ubuntu.com/p/q2DVwVh6hr/plain/

If you fix this issue, please add the following tag to the commit:

Reported-by: Hypericum <hypericumperforatum4444@xxxxxxxxx>

I think fs/proc/task_mmu.c visits the variable mm without locking and
another mmap visits mm->hiwater_vm resulting in a data race.

reproducer log: https://paste.ubuntu.com/p/Sp6RDWKDfy/plain/
reproducer report:
==================================================================
BUG: KCSAN: data-race in task_mem / unmap_region

write to 0xffff8881095008b0 of 8 bytes by task 3712 on cpu 4:
 update_hiwater_rss include/linux/mm.h:2102 [inline]
 unmap_region+0x12b/0x1e0 mm/mmap.c:2648
 __do_munmap+0xe6e/0x12b0 mm/mmap.c:2883
 __vm_munmap mm/mmap.c:2906 [inline]
 __do_sys_munmap+0x9f/0x160 mm/mmap.c:2932
 __se_sys_munmap mm/mmap.c:2928 [inline]
 __x64_sys_munmap+0x2d/0x40 mm/mmap.c:2928
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x44/0xd0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff8881095008b0 of 8 bytes by task 1512 on cpu 7:
 task_mem+0xfb/0x3d0 fs/proc/task_mmu.c:50
 proc_pid_status+0x890/0x14d0 fs/proc/array.c:438
 proc_single_show+0x84/0x100 fs/proc/base.c:778
 seq_read_iter+0x2e3/0x930 fs/seq_file.c:230
 seq_read+0x234/0x280 fs/seq_file.c:162
 vfs_read+0x1e6/0x730 fs/read_write.c:479
 ksys_read+0xd9/0x190 fs/read_write.c:619
 __do_sys_read fs/read_write.c:629 [inline]
 __se_sys_read fs/read_write.c:627 [inline]
 __x64_sys_read+0x3e/0x50 fs/read_write.c:627
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x44/0xd0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0x000000000000065b -> 0x0000000000000662

Reported by Kernel Concurrency Sanitizer on:
CPU: 7 PID: 1512 Comm: systemd-journal Not tainted 5.16.0-rc8+ #11
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
1.13.0-1ubuntu1.1 04/01/2014
==================================================================